Among electronic devices, there are some devices or the like utilizing an MEMS technique that dramatically increase performance by being mounted into a vacuum-sealed package.
For example, in a thermal infrared sensor, heat dissipation from a sensor structure largely affects performance, and thus, it is desired to reduce heat conduction through the atmosphere as much as possible. Moreover, some of acceleration sensors and angler speed sensors are, for a specific purpose, required to be operated in a vacuum environment in order to reduce damping by the atmosphere (mechanical resistance of the air). Furthermore, in a radio-frequency device, vacuum sealing is performed for the purpose of increasing properties of a resonator, which is a vibrating device, or for the purpose of preventing sticking of a device involving mechanical contact such as a switch (sticking of a movable structure to a substrate).
For reliability test or self-diagnosis (a function included in a production to check that the product is normally operating) of such an electronic device requiring the vacuum sealing, pressure (atmospheric pressure) inside the package containing the device needs to be measured.
As pressure measurement of gas in a minute space such as the package of the electronic device, there have been a micro vacuum gauge by a heat conduction method, and the like, in which a minute heater that can be manufactured in a semiconductor process is held adiabatically (in a state where heat conductance is low) from a semiconductor substrate, and pressure dependency of the heat conduction of this gas is utilized (e.g., refer to Non-Patent Document 1).
Non-Patent Document 1: A. W. van Herwaarden and P. M. Sarro, J. Vac. Sci. Technol., Vol. A5, No. 4, pp. 2454-2457, 1987.
A conventional micro vacuum gauge 90 by a heat conduction method, as seen from a basic structure shown in FIGS. 16 and 17, has a floating structure 92 held above a hollow space S formed in a semiconductor substrate 91, and in the floating structure 92, a heater 93 and a temperature sensor 94 are arranged. While in an example shown in the figures, the floating structure 92 is supported by a supporting structure 98 so as to be supported at almost the same height as that of an upper surface of the semiconductor substrate 91, a structure can also be employed, in which the floating structure 92 is elevated upward from the upper surface of the semiconductor substrate 91.
Next, operation of the micro vacuum gauge 90 is described. When a current is passed through the heater 93 to generate Joule heat, a temperature of the floating structure 92 rises. The rise in temperature is decided by a power supplied to the heater 93, and a heat loss (transmitted heat quantity) flowing from the floating structure 92 to the semiconductor substrate 91 serving as a heat sink and peripheral structures not shown. This heat loss is due to three factors of heat conduction through gas surrounding the floating structure 92, heat conduction through the supporting structure 98, and heat radiation from the floating structure 92.
A relationship between a heat loss Q from the floating structure 92 and a pressure P of the ambient gas can be considered by dividing into three regions R1, R2, R3, as shown in FIG. 18 in which a vertical axis and a horizontal axis are both logarithmic axes. The region R1 is a region where the fact that since it is a region of a high pressure, an average free path of the gas is shorter than a length of the hollow space S so that the heat conduction of the gas does not depend on the pressure P is reflected, and thus, the heat loss Q hardly depends on the pressure P although the region has slight pressure dependency by an effect of convection. The region R2 is a region in which since the average free path of the gas is longer than the length of the hollow space S, the heat conduction through the gas is proportional to the pressure (molecular density) of the gas, and with reduction in pressure, the heat loss is reduced. The region R3 is a low-pressure region with the heat loss Q at a lowest level, in which the heat loss Q does not depend on the pressure P of the gas, and the heat loss by the heat conduction through the supporting structure 98 and the heat loss by the heat emission (heat radiation) from the floating structure 92 are added.
Taking into consideration the above-described dependency of the heat loss on the pressure of the ambient gas, a temperature measured in the temperature sensor 94 does not change in the region R1 when a constant current is supplied to the heater 93, so that the pressure cannot be measured, while in the region R2, since the heat conduction through the gas is reduced with the decrease in pressure, the temperature rises, and in the region R3, the pressure dependency is again eliminated, so that the temperature becomes constant. Accordingly, the conventional micro vacuum gauge 90 by the heat conduction method can measure the pressure only in the range of the region R2, and a lower limit thereof is about 1 Pa.